There are a number of fields in which it is desirable to locate and identify a target substance within a medium. In the medical field, it is desirable to optically detect cancerous tissue, plaque on arterial walls, and other selected substances.
One technique for diagnosing the presence of a tumor uses photodynamic sensitizers, for example dyes such as hematoporphyrin derivative (HPD), Rhodamine 123, and merocyanine, which are injected into the human or animal body, where they are selectively retained by cancerous tissue. After a predetermined period of time, typically two or three days after injection, significantly high levels of photodynamic sensitizer are retained in malignant tissue relative to levels in normal tissue. When irradiated with ultraviolet or short wavelength visible light, the higher concentrations of photodynamic sensitizer within the malignant tissue exhibit a fluorescence at a color substantially different from the light pink appearance of normal tissue. For example, HPD exhibits a bright red fluorescence. See Baumgarthner et al., Photochem and Photobiol., Vol. 46, pp. 1-11 (1987); and Anderson et al., Med. Phys. Vol. 14, pp. 623-636 (1987).
Presently, however, it is difficult to optically detect malignant tissue that does not lie on the surface of the tissue being examined. The deeper the target lies within the tissue, the greater the amount of dye which is activated by the radiating beam of light. In other words, a deeply penetrating beam induces fluorescence along its entire path. Such devices therefore suffer from a low signal-to-noise ratio.
In an entirely different application, there are a number of systems which measure the velocity of moving particles using two beams which are at slightly different frequencies from each other. The intersection of the beams produces a beat frequency, also known as a heterodyne effect. The velocity of moving particles is determined by measuring the Doppler shift induced in the beat frequency; in other words, the greater the particle velocity, the greater the observed frequency is shifted from the original beat frequency. Several systems then use the Doppler shift information to determine particle concentration. Such systems, however, are designed to observe particles moving through a fluid.